Currently, no short term (less than 2 days) test method exists to evaluate the long-term corrosion protection afforded by a protective coating from a coating composition, such as automotive OEM or automotive refinish coating compositions, applied over a metal substrate, such as automotive body. The current standard test methods rely primarily on environmental chamber exposure, followed by visual and mechanical testing of the metal with its protective coating. This kind of testing is long (up to 40 days or more exposure time), subjective, highly dependent on the exposure geometry, and on the person doing the evaluation. Consequently, these methods are not very reproducible. The corrosion resistance data is qualitative, and therefore the relative performance of an acceptable coating cannot be easily determined. Any new test method must correlate well with the traditional, the accepted, standard environmental chamber test methods, must be reproducible, and must supply a qualitative and quantitative ranking of the unknown direct-to-metal (DTM) corrosion resistant coating.
The experimental corrosion test methods have been reported for reducing the test duration. These methods primarily utilize electrochemical impedance spectroscopy (EIS) or AC impedance technology. Since these AC impedance based methods typically only offer a more sensitive tool for detecting corrosion at an early stage of exposure time, the corrosion process itself is not accelerated by these methods. Therefore, these methods still require relatively long exposure times before the meaningful data can be obtained. The length of time needed to get meaningful corrosion data approaches that of the standard methods. More importantly, the corrosion resistance data obtained by these methods, particularly during the initial exposure time, are primarily dictated by the intrinsic defects of the coatings. These intrinsic defects generally produced during the preparation of coated samples are not necessarily related to the actual performance of the coatings. Misleading information could be obtained if the data are not analyzed correctly. Consequently, the standard convention methods are still favored. Therefore, a need still exists for a device and a process that not only accelerates the corrosion of protectively coated metal substrates but also mimics the corrosion typically seen in working environments, such as those experienced by bodies of automobiles during use.